The invention relates generally to the definition of a number of data tracks on a recording medium. In particular, the invention relates to the definition of a plurality of concentric data tracks on a magnetic disc, the accessing of these data tracks through the use of an embedded servo control system and a technique for writing servo data.
Record discs are commonly used for storing data in computers and other data processing systems. Such record discs are provided on both faces with a layer of magnetizable material for recording data in the formm of a plurality of concentric tracks. Some disc systems or disc files as they are frequently called, include a plurality of such record discs stacked on a common spindle and driven by a common drive whereas others include only one record disc. The discs are rotated at a constant speed and transducers or heads capable of either reading or writing data on the discs are flown over the disc surfaces on an air film. Although there are a large number of tracks on each disc surface, generally only one or two read/write heads are provided for each disc surface. Thus, the transducers or heads must be translatable across the surface of the discs and must include means for very accurately positioning or aligning the heads with a preselected data track. Addressing data stored on the disc is accomplished by means for selecting a read/write head, specifying the track position and specifying the segment or sector of the track to be accessed. The velocity and position of a read/write head positioner is controlled by a servo control circuit. The read/write head positioner is controlled by a servo control circuit. The read/write heads associated with each surface of each disc are generally positioned in unison by a single positioner. Once properly positioned to the addressed track, data transferred to or from the track is affected upon selecting the head associated with the specific disc surface bearing the track to be accessed.
The art of digital recording on a magnetic disc has advanced to the point where data is being recorded in ever increasing densities on the magnetic disc surfaces. There are a number of approaches which have been taken in compacting the data on a magnetic disc. One approach has been to devise novel digital coding techniques which increase the information content in each magnetic recording on the disc. Another approach has been to increase the number of concentric data tracks on a disc surface to the point where the tracks are very densely packed. This latter approach has led to the development of increasingly sophisticated servo systems that are capable of successively positioning a transducer or head in closely spaced increments over a magnetic disc. The very earliest track following servo systems actually did not follow the data tracks. In these earliest systems, position signals were derived from position transducers which were independent from the read/write heads. In some cases, an optical pickup was used to generate a position signal as an arm carrying the read/write heads radially traversed the disc. As track densities increased, optical systems were not accurate enough and magnetic transducers which traversed a dedicated servo surface were used. In some cases optical systems were used to coarsely position the heads and the magnetic transducer was used to finely position the heads. In systems employing a dedicated servo surface position information was magnetically recorded on a dedicated servo surface and read by a magnetic recording head which was separate from the read/write heads and was generally referred to as a servo head. As positioning systems further progressed, various schemes for putting the data on the dedicated servo surface were used to increase track densities. All of these earlier positioning systems are referred to as continuous data track following systems since position information is always continuously available from the dedicated servo head. However, these prior art continuous data track following systems were far from optimum track following systems since these systems did not actually follow the data track that they were intended to read. As the distance between data tracks became closer and closer, variations in the relative positions of the separate read/write heads and servo heads become much more significant. Eventually changes in the relative position of these transducers due to thermal effects or other purely mechanical problems became the factors limiting track densities in prior art continuous data track following systems.
Other continuous data track following servo systems have employed a special recording medium having overlapping magnetizable layers with different coercivities.
A natural breaking point in the development of disc track following systems ocurred with the development of sample data track following systems. Sample data track following systems attempt to follow the actual data track that is being read with the read/write head. This is generally accomplished by interposing bursts of servo control data between sectors on the recording media containing the informational data which is being read or written. However, many previous sample data systems did not give up the older continuous data type positioning system. In other words, both the older continuous data system and the newer sample data system are used to control the read/write heads. Typically, an optical pickup or a dedicated servo surface and dedicated servo head are used to coarsely adjust the read/write head and a sample data system is employed to finely adjust the read/write head, the servo information for the sample data system coming directly from the track that is being read by the read/write head. Problems with this type of track following system are related to its cost and complexity and the manner in which servo data is coded on the magnetic disc.
As interest increased in embedded servo track following systems, a variety of schemes such as "di bit", "tri bit", "quad bit" and variations thereof developed for recording servo data on a recording medium that is later read by the read/write head, along with the data being read or recorded, to create a position signal. While some of these sample data systems eliminate the dedicated servo surface and dedicated servo head previously necessary for coarse positioning of the read/write head, most of these prior art sample data techniques record servo data with an amplitude sensitive scheme. Amplitude demodulation of servo data presents several problems all related to the fact that in an amplitude modulated system an automatic gain control (AGC) or equivalent circuitry is required. In amplitude modulated systems, the first pulses of the servo data must be used to normalize the signal. In the stream of servo data, the first pulses all appear at the same amplitude since they are recorded in adjacent tracks in the same direction. Sets of these sequential pulses are recorded differently on adjacent servo data tracks and the amplitude of a second set of pulses associated with an adjacent servo track is then compared to the first set of pulses to determine the radial position of the head relative to adjacent servo data tracks. The circuitry necessary for accomplishing these functions is quite different from the circuitry used to read the informational data stream. Thus, a not insubstantial amount of additional circuitry is required to demodulate the position signal. Furthermore, a significant number of pulses are required for the AGC to determine an average pulse and this substantially increases the overhead of the system, or the amount of the recording media on the disc dedicated to the servo data.
The principal exception to amplitude modulated sample data systems involves the use of adjacent servo tracks which generate waveforms of different frequencies. In this case control circuitry is provided to frequency demodulate the signals generated by adjacent servo data tracks, generate a difference signal when the modulated signal and each servo waveform signal are multiplied and then detect the amplitude of each different signal, thereby sensing the relative position of the transducer and the data track. However, these techniques also require substantial circuitry of a different type from the circuitry normally used to read the data stream from the read/write head and require a substantial overhead. In most prior art sample data track following servo systems, at least 15 percent of the disc surface is dedicated to servo data.
Another exception to the amplitude modulated sample data systems and to the previously discussed frequency modulated system is the system disclosed in U.S. patent application Ser. No. 123,501 filed Feb. 22, 1980 entitled EMBEDDED SERVO TRACK FOLLOWING SYSTEM and assigned to the same assignee now abandoned. The system disclosed in this application features a technique for coding embedded servo data that does not require substantial circuitry of a different type from the circuitry normally used to read the data stream coming from the read/write head. Thus, a position signal is generated with substantially the same circuitry used to read and write data on the disc, achieving a substantial savings in cost. In this system, alternately spaced odd and even servo data tracks are provided which generate net positive and net negative DC signals when the read/write head is positioned thereover. The servo data tracks are written at a frequency which is similar to the frequency of data normally read and written with the read/write heads. Two frequencies are written on each of the odd and even servo data tracks, one frequency being impressed upon the other. However, the series of transitions that comprise the odd servo data tracks are provided with a different order or direction from the series of transitions which comprise the even servo data tracks. More specifically, each of the odd and even tracks comprise a series of equally spaced transition pairs, each of the transition pairs comprising first and second equally spaced oppositely directed transitions. Each of the transition pairs disposed in the even track are provided with an identical order and each of the transition pairs disposed in an adjacent odd track are provided with an order identically opposite of the order of the transition pairs disposed in the even track. The transitions of each transition pair are provided with a spacing that is equal but less than the spacing of the transition pairs themselves. Adjacent odd and even servo tracks thus formed, generate waveforms having DC components equal in magnitude but opposite in polarity such that the DC components of the odd and even servo data tracks cancel each other when a read/write head is disposed therebetween. Servo control means is provided for sensing the output of the read/write heads and selectively energizing a positioning means for nulling the output of the read/write head and accurately centering the same on an informational data track that is disposed therebetween.
In the prior art, embedded servo data is usually written with a complex, mechanical/optical technique that involves a sophisticated lead screw actuated mechanical device which is pressed against the existing read/write head positioner for establishing the position of the read/write heads and a laser interferometry technique for precisely determining the position of the heads. Not only is this mechanism expensive, but the method of writing embedded servo with this mechanism is time consuming. Furthermore, in the past no provision has been made for the changing performance characteristics of the read/write heads as they radially traverse the disc. Since the performance of the read/write heads is reduced as the head approaches the interior of the disc, in the prior art a constant track density is chosen which is limited by the performance characteristics of the heads on the interior of the recording surface of the disc.